Tissue manufacturers that can deliver the highest quality product at the lowest cost have a competitive advantage in the marketplace. A key component in determining the cost and quality of a tissue product is the manufacturing process utilized to create the product. For tissue products, there are several manufacturing processes available including conventional dry crepe, through air drying (TAD), or “hybrid” technologies such as Valmet's NTT and QRT processes, Georgia Pacific's ETAD, and Voith's ATMOS process. Each has differences as to installed capital cost, raw material utilization, energy cost, production rates, and the ability to generate desired attributes such as softness, strength, and absorbency.
Conventional manufacturing processes include a forming section designed to retain the fiber, chemical, and filler recipe while allowing the water to drain from the web. Many types of forming sections, such as inclined suction breast roll, twin wire C-wrap, twin wire S-wrap, suction forming roll, and Crescent formers, include the use of forming fabrics.
Forming fabrics are woven structures that utilize monofilaments (such as yarns or threads) composed of synthetic polymers (usually polyethylene, polypropylene, or nylon). A forming fabric has two surfaces, the sheet side and the machine or wear side. The wear side is in contact with the elements that support and move the fabric and are thus prone to wear. To increase wear resistance and improve drainage, the wear side of the fabric has larger diameter monofilaments compared to the sheet side. The sheet side has finer yarns to promote fiber and filler retention on the fabric surface.
Different weave patterns are utilized to control other properties such as: fabric stability, life potential, drainage, fiber support, and clean-ability. There are three basic types of forming fabrics: single layer, double layer, and triple layer. A single layer fabric is composed of one yarn system made up of cross direction (CD) yarns (also known as shute yarns) and machine direction (MD) yarns (also known as warp yarns). The main issue for single layer fabrics is a lack of dimensional stability. A double layer forming fabric has one layer of warp yarns and two layers of shute yarns. This multilayer fabric is generally more stable and resistant to stretching. Triple layer fabrics have two separate single layer fabrics bound together by separated yarns called binders. Usually the binder fibers are placed in the cross direction but can also be oriented in the machine direction. Triple layer fabrics have further increased dimensional stability, wear potential, drainage, and fiber support than single or double layer fabrics.
The manufacturing of forming fabrics includes the following operations: weaving, initial heat setting, seaming, final heat setting, and finishing. The fabric is made in a loom using two interlacing sets of monofilaments (or threads or yarns). The longitudinal or machine direction threads are called warp threads and the transverse or machine direction threads are called shute threads. After weaving, the forming fabric is heated to relieve internal stresses to enhance dimensional stability of the fabric. The next step in manufacturing is seaming. This step converts the flat woven fabric into an endless forming fabric by joining the two MD ends of the fabric. After seaming, a final heat setting is applied to stabilize and relieve the stresses in the seam area. The final step in the manufacturing process is finishing, whereby the fabric is cut to width and sealed.
There are several parameters and tools used to characterize the properties of the forming fabric: mesh and count, caliper, frames, plane difference, open area, air permeability, void volume and distribution, running attitude, fiber support, drainage index, and stacking. None of these parameters can be used individually to precisely predict the performance of a forming fabric on a paper machine, but together the expected performance and sheet properties can be estimated. Examples of forming fabrics designs can be viewed in U.S. Pat. Nos. 3,143,150, 4,184,519, 4,909,284, and 5,806,569.
In a conventional dry crepe process, after web formation and drainage (to around 35% solids) in the forming section (assisted by centripetal force around the forming roll and, in some cases, vacuum boxes), a web is transferred
from the forming fabric to a press fabric upon which the web is pressed between a rubber or polyurethane covered suction pressure roll and Yankee dryer. The press fabric is a permeable fabric designed to uptake water from the web as it is pressed in the press section. It is composed of large monofilaments or multi-filamentous yarns, needled with fine synthetic batt fibers to form a smooth surface for even web pressing against the Yankee dryer. Removing water via pressing reduces energy consumption.
In a conventional TAD process, rather than pressing and compacting the web, as is performed in conventional dry crepe, the web undergoes the steps of imprinting and thermal pre-drying. Imprinting is a step in the process where the web is transferred from a forming fabric to a structured fabric (or imprinting fabric) and subsequently pulled into the structured fabric using vacuum (referred to as imprinting or molding). This step imprints the weave pattern (or knuckle pattern) of the structured fabric into the web. This imprinting step increases softness of the web, and affects smoothness and the bulk structure. The manufacturing method of an imprinting fabric is similar to a forming fabric (see U.S. Pat. Nos. 3,473,576, 3,573,164, 3,905,863, 3,974,025, and 4,191,609 for examples) except for an additional step if an overlaid polymer is utilized.
Imprinting fabrics with an overlaid polymer are disclosed in U.S. Pat. Nos. 5,679,222, 4,514,345, 5,334,289, 4,528,239 and 4,637,859. Specifically, these patents disclose a method of forming a fabric in which a patterned resin is applied over a woven substrate. The patterned resin completely penetrates the woven substrate. The top surface of the patterned resin is flat and openings in the resin have sides that follow a linear path as the sides approach and then penetrate the woven structure.
U.S. Pat. Nos. 6,610,173, 6,660,362, 6,998,017, and European Patent No. EP 1 339 915 disclose another technique for applying an overlaid resin to a woven imprinting fabric.
After imprinting, the web is thermally pre-dried by moving hot air through the web while it is conveyed on the structured fabric. Thermal pre-drying can be used to dry the web to over 90% solids before the web is transferred to a steam heated cylinder. The web is then transferred from the structured fabric to the steam heated cylinder though a very low intensity nip (up to 10 times less than a conventional press nip) between a solid pressure roll and the steam heated cylinder. The portions of the web that are pressed between the pressure roll and steam cylinder rest on knuckles of the structured fabric; thereby protecting most of the web from the light compaction that occurs in this nip. The steam cylinder and an optional air cap system, for impinging hot air, then dry the sheet to up to 99% solids during the drying stage before creping occurs. The creping step of the process again only affects the knuckle sections of the web that are in contact with the steam cylinder surface. Due to only the knuckles of the web being creped, along with the dominant surface topography being generated by the structured fabric, and the higher thickness of the TAD web, the creping process has much smaller effect on overall softness as compared to conventional dry crepe. After creping, the web is optionally calendered and reeled into a parent roll and ready for the converting process. Some TAD machines utilize fabrics (similar to dryer fabrics) to support the sheet from the crepe blade to the reel drum to aid in sheet stability and productivity. Patents which describe creped through air dried products include U.S. Pat. Nos. 3,994,771, 4,102,737, 4,529,480, and 5,510,002.
The TAD process generally has higher capital costs as compared to a conventional tissue machine due to the amount of air handling equipment needed for the TAD section. Also, the TAD process has a higher energy consumption rate due to the need to burn natural gas or other fuels for thermal pre-drying. However, the bulk softness and absorbency of a paper product made from the TAD process is superior to conventional paper due to the superior bulk generation via structured fabrics, which creates a low density, high void volume web that retains its bulk when wetted. The surface smoothness of a TAD web can approach that of a conventional tissue web. The productivity of a TAD machine is less than that of a conventional tissue machine due to the complexity of the process and the difficulty of providing a robust and stable coating package on the Yankee dryer needed for transfer and creping of a delicate a pre-dried web.
UCTAD (un-creped through air drying) is a variation of the TAD process in which the sheet is not creped, but rather dried up to 99% solids using thermal drying, blown off the structured fabric (using air), and then optionally calendered and reeled. U.S. Pat. No. 5,607,551 describes an uncreped through air dried product.
A process/method and paper machine system for producing tissue has been developed by the Voith company and is marketed under the name ATMOS. The process/method and paper machine system has several variations, but all involve the use of a structured fabric in conjunction with a belt press. The major steps of the ATMOS process and its variations are stock preparation, forming, imprinting, pressing (using a belt press), creping, calendering (optional), and reeling the web.
The stock preparation step of the ATMOS process is the same as that of a conventional or TAD machine. The forming process can utilize a twin wire former (as described in U.S. Pat. No. 7,744,726), a Crescent Former with a suction Forming Roll (as described in U.S. Pat. No. 6,821,391), or a Crescent Former (as described in U.S. Pat. No. 7,387,706). The former is provided with a slurry from the headbox to a nip formed by a structured fabric (inner position/in contact with the forming roll) and forming fabric (outer position). The fibers from the slurry are predominately collected in the valleys (or pockets, pillows) of the structured fabric and the web is dewatered through the forming fabric. This method for forming the web results in a bulk structure and surface topography as described in U.S. Pat. No. 7,387,706 (FIGS. 1-11). After the forming roll, the structured and forming fabrics separate, with the web remaining in contact with the structured fabric.
The web is now transported on the structured fabric to a belt press. The belt press can have multiple configurations. The press dewaters the web while protecting the areas of the sheet within the structured fabric valleys from compaction. Moisture is pressed out of the web, through the dewatering fabric, and into the vacuum roll. The press belt is permeable and allows for air to pass through the belt, web, and dewatering fabric, and into the vacuum roll, thereby enhancing the moisture removal. Since both the belt and dewatering fabric are permeable, a hot air hood can be placed inside of the belt press to further enhance moisture removal. Alternately, the belt press can have a pressing device which includes several press shoes, with individual actuators to control cross direction moisture profile, or a press roll. A common arrangement of the belt press has the web pressed against a permeable dewatering fabric across a vacuum roll by a permeable extended nip belt press. Inside the belt press is a hot air hood that includes a steam shower to enhance moisture removal. The hot air hood apparatus over the belt press can be made more energy efficient by reusing a portion of heated exhaust air from the Yankee air cap or recirculating a portion of the exhaust air from the hot air apparatus itself.
After the belt press, a second press is used to nip the web between the structured fabric and dewatering felt by one hard and one soft roll. The press roll under the dewatering fabric can be supplied with vacuum to further assist water removal. This belt press arrangement is described in U.S. Pat. Nos. 8,382,956 and 8,580,083, with FIG. 1 showing the arrangement. Rather than sending the web through a second press after the belt press, the web can travel through a boost dryer, a high pressure through air dryer, a two pass high pressure through air dryer or a vacuum box with hot air supply hood. U.S. Pat. Nos. 7,510,631, 7,686,923, 7,931,781, 8,075,739, and 8,092,652 further describe methods and systems for using a belt press and structured fabric to make tissue products each having variations in fabric designs, nip pressures, dwell times, etc., and are mentioned here for reference. A wire turning roll can be also be utilized with vacuum before the sheet is transferred to a steam heated cylinder via a pressure roll nip.
The sheet is now transferred to a steam heated cylinder via a press element. The press element can be a through drilled (bored) pressure roll, a through drilled (bored) and blind drilled (blind bored) pressure roll, or a shoe press. After the web leaves this press element and before it contacts the steam heated cylinder, the % solids are in the range of 40-50%. The steam heated cylinder is coated with chemistry to aid in sticking the sheet to the cylinder at the press element nip and also to aid in removal of the sheet at the doctor blade. The sheet is dried to up to 99% solids by the steam heated cylinder and an installed hot air impingement hood over the cylinder. This drying process, the coating of the cylinder with chemistry, and the removal of the web with doctoring is explained in U.S. Pat. Nos. 7,582,187 and 7,905,989. The doctoring of the sheet off the Yankee, i.e., creping, is similar to that of TAD with only the knuckle sections of the web being creped. Thus, the dominant surface topography is generated by the structured fabric, with the creping process having a much smaller effect on overall softness as compared to conventional dry crepe. The web is now calendered (optional), slit, reeled and ready for the converting process.
The ATMOS process has capital costs between that of a conventional tissue machine and a TAD machine. It uses more fabrics and a more complex drying system compared to a conventional machine, but uses less equipment than a TAD machine. The energy costs are also between that of a conventional and a TAD machine due to the energy efficient hot air hood and belt press. The productivity of the ATMOS machine has been limited due to the inability of the novel belt press and hood to fully dewater the web and poor web transfer to the Yankee dryer, likely driven by poor supported coating packages, the inability of the process to utilize structured fabric release chemistry, and the inability to utilize overlaid fabrics to increase web contact area to the dryer. Poor adhesion of the web to the Yankee dryer has resulted in poor creping and stretch development which contributes to sheet handling issues in the reel section. The result is that the output of an ATMOS machine is currently below that of conventional and TAD machines. The bulk softness and absorbency is superior to conventional, but lower than a TAD web since some compaction of the sheet occurs within the belt press, especially areas of the web not protected within the pockets of the fabric. Also, bulk is limited since there is no speed differential to help drive the web into the structured fabric as exists on a TAD machine. The surface smoothness of an ATMOS web is between that of a TAD web and a conventional web primarily due to the current limitation on use of overlaid structured fabrics.
The ATMOS manufacturing technique is often described as a hybrid technology because it utilizes a structured fabric like the TAD process, but also utilizes energy efficient means to dewater the sheet like the conventional dry crepe process. Other manufacturing techniques which employ the use of a structured fabric along with an energy efficient dewatering process are the ETAD process and NTT process. The ETAD process and products are described in U.S. Pat. Nos. 7,399,378, 7,442,278, and 7,494,563. The NTT process and products are described in US Patent Application Publication No. 2011/0180223 A1, and US Patent Application Publication No. 2010/0065234 A1. The QRT process is described in US Patent Application Publication No. 2008/0156450 A1 and U.S. Pat. No. 7,811,418. A structuring belt manufacturing process used for the NTT, QRT, and ETAD imprinting process is described in U.S. Pat. No. 8,980,062 and U.S. Patent Application Publication No. US 2010/0236034.
The NTT process involves spirally winding strips of polymeric material, such as industrial strapping or ribbon material, and adjoining the sides of the strips of material using ultrasonic, infrared, or laser welding techniques to produce an endless belt. Optionally, a filler or gap material can be placed between the strips of material and melted using the aforementioned welding techniques to join the strips of materials. The strips of polymeric material are produced by an extrusion process from any polymeric resin such as polyester, polyamide, polyurethane, polypropylene, or polyether ether ketone resins. The strip material can also be reinforced by incorporating monofilaments of polymeric material into the strips during the extrusion process or by laminating a layer of woven polymer monofilaments to the non-sheet contacting surface of a finished endless belt composed of welded strip material. The endless belt can have a textured surface produced using processes such as sanding, graving, embossing, or etching. The belt can be impermeable to air and water, or made permeable by processes such as punching, drilling, or laser drilling. Examples of structuring belts used in the NTT process can be viewed in International Publication Number WO 2009/067079 A1 and US Patent Application Publication No. 2010/0065234 A1.
As shown in the aforementioned discussion of tissue papermaking technologies, the fabrics or belts utilized are critical in the development of the tissue web structure and topography which, in turn, are instrumental in determining the quality characteristics of the web such as softness (bulk softness and surfaces smoothness) and absorbency. The manufacturing process for making these fabrics has been limited to weaving a fabric (primarily forming fabrics and structured fabrics) or a base structure and needling synthetic fibers (press fabrics) or overlaying a polymeric resin (overlaid structured fabrics) to the fabric/base structure, or welding strips of polymeric material together to form an endless belt.
Conventional overlaid structures require application of an uncured polymer resin over a woven substrate where the resin completely penetrates through the thickness of the woven structure. Certain areas of the resin are cured and other areas are uncured and washed away from the woven structure. This results is a fabric where airflow through the fabric is only possible in the Z-direction. Thus, in order for the web to dry efficiently, only highly permeable fabrics can be utilized, meaning the amount of overlaid resin applied needs to be limited. If a fabric of low permeability is produced in this manner, then drying efficiency is significantly reduced, resulting in poor energy efficiency and/or low production rates as the web must be transported slowly across the TAD drums or ATMOS drum for sufficient drying. Similarly, a welded polymer structuring layer is extremely planar and provides an even surface when laminating to a woven support layer (FIG. 9), which results in little if any air channels in the X-Y plane.